The present invention relates to a screwdriver comprising a screwdriver shank which is selectively retainable in an automatic state or in a latched state to automatically tighten or loosen screws merely when the handle portion of the screwdriver is pushed against a screw, or to manually tighten or loosen screws when the handle portion is turned respectively.
Ideally screwdrivers of this type should have the following features:
First, the screwdriver shank should be axially movable forward and backward while in rotation.
Second, the shank should be capable of being reliably latched in its retracted position.
Third, the shank should be capable of being so latched with a small force.
Fourth, the shank should be reliably releasable from its latched position.
Fifth, the latch member should be free of fatigue and other defects for years.
With screwdrivers of the type described above, heretofore widely used, a latch pawl biased by a leaf spring is engaged in or disengaged from an annular groove formed in a rear end portion of the screwdriver shank to retain the shank in a latched state or alternatively in an automatic state.
To ensure the engagement or disengagement of the latch pawl, the leaf spring must have a biasing force sufficient to reliably release the latch pawl from the annular groove. However, when the spring has an increased elastic force, the latch pawl requires a greater force for engagement into the annular groove, and the spring, even if having an enhanced force, becomes fatigued and weakened during years of use, thus failing to assure a proper change-over action.